Dr. Lanigan, in his work, An ecclesiastical history of Ireland, Chapter IX, relates of St. Tigernach, as follows:
"St. Maccarthen of Clogher, whose history I have been obliged to anticipate, died, as already stated, in the year 506; and, as some say, on the 24th of March. He was succeeded by St. Tigernach, who fixed his see or residence at Cluaneois (Clunes or Clones) in the county of Monaghan, still retaining government of the church of Clogher, for which reason he was surnamed Ferdachrioch, or the man of two districts. He is said to have been of a princely family, grandson, by his mother, of a king Echodius, and to have had St. Brigid for godmother, through whose recommendation he was raised to the episcopal dignity. He had received his clerical education, as we are told, in the monastery of Rosnat in Great Britain under the holy abbot Monennus, and, it seems founded that of Clones before he was appointed bishop."
Dr. Lanigan comments on the association of St. Brigid with St. Tigernach:
"If this narrative deserves credit, we must suppose that St. Brigid's standing as godmother for Tigernach was in her younger days, and, at least 30 years before A.D. 506. On this occasion it is observed that whoever was recommended for the episcopacy by St. Brigid, was immediately approved of and chosen by the clergy and people. (Compare with what has been said about Conlaeth of Kildare Chap. VIII, No. 10)"
Dr. Lanigan, in a passage that is a model of his scholarship and his prose, speculates upon the location of Rosnat Abbey:
"Where was that monastery of Rosnat? Neither the Monasticon Anglicanum, Stevens, Tanner, Nasmith, nor Camden have, as far as I could discover, a word about it, although it is often mentioned in the Acts of some Irish saints. In those of Tigernach, quoted by Colgan (ib.) it is observed that it was otherwise called Alba, or white. Colgan hence concludes that it was no other than the famous monastery of Bangor or Banchor near the river Dee a few miles from Chester, which must be carefully distinguished from the present episcopal town Bangor, which lies far to the West of where the monastery stood. (See Usher, p. 183.) His chief argument is that Ban, in Irish, signifies white, and so Ban-chor was the same as white choir. But, waving certain doubts concerning the said monastery having existed at that early period, it is to be recollected that Ban has not that signification in the British language, which is that to be looked to in this inquiry. I suspect that Rosnat or Alba was the celebrated see called Candida casa or White house, now Whitethorn. (See Not. 149, to Chap. 1.) The illustrious Ninia or Ninian had founded that see in the 5th century, and there can be no doubt of an ecclesiastical school having been established there. (See Usher, p. 661. seqq.) When we read of Nennio being the bishop, to whom some Irish students were sent, this, I believe, must be understood as originally meaning that they were sent to the school held in the see or Nennio or Ninia, who was dead before Tigernach or Finnian could have repaired thither. And in fact Finnian's master is called Mugentius, and what is very remarkable, the place Candida (AA. SS. p. 634). The master of Endeus of Arran, who is also said to have been at that school, is called not Nennio but Mansenus. Let me add that Candida casa lay very convenient for students from the North of Ireland; and it is worth observing, that of those, who are spoken of as having studied at Rosnat or Alba, scarcely one is to be found that was not a native of Ulster. There is a village and parish in Dumbartonshire, called Roseneath, anciently Rossnachioch, (Stat. Acct. of Scotland, Vol. IV. p. 71.) But there is no mention of a monastery having been there."
He goes on to quote from the Four Masters regarding the death of the Saint:
"An. 548 (549) St. Tigernac, bishop of Cluaineois, died on the 4th of April."
The Martyrology of Donegal gives his death as 4th April, 548, and gives something of his descent as follows:
"Bishop of Cluaoi-eois in Fera-Manach, or it is between Fera-Manach and Oirghialla Cluain-eois is. Tighernach is of the race of Cathaoir Mór, Monarch of Erinn, of the Leinstermen. Dearfraoich, daughter of Eochaidh, son of Criomhthann, king of Oirchiall, was his mother."
In the Life of St. Tighernach, quoted in Butler's Lives of the Irish Saints, it is stated that, while passing through Kildare, city of St. Brigid, with his foster-father, Cormac, who may well have been his maternal grandfather, the future saint was baptised by St. Conleth. Butler continues:
"From the foregoing narrative, Bollandus infers, that as Conlaid had been a bishop, when he baptized St. Tighernach, his elevation to the episcopal rank must have been accomplished previous to A.D. 480. For, St. Maccarthen died in the year 506; and, he was immediately succeeded in the See of Clogher by St. Tighernach. Supposing correctness in the foregoing account, it is conjectured, his baptism must have taken place, at least thirty years before the latter date, and during the younger days of his godmother, St. Brigid."
St. Tigernach of Clones and Clogher, pray for us!